you'll find two discussions from Atomicsoda on this subject. In the first I gave some details and a few helpful links (there are much more to be found, and probably better also, but the info from the old forum has gone). In the second Bob Proffit shows you have to have your BIOS password ready also, if that write-protects the bootsector, and you gave it a password.
The answer to your question is something like: passwords, settings, applications (cd, setup.exe or know where to download it), personal data, downloaded music. In fact you have to know about about every piece of data and program on your machine that didn't come from the Windows 98 CD.
It would be painful, to name a thing, if you had saved your Internet-access (dial-up or cable) password, and could't find the letter it was in (which you received maybe a few years ago) when setting up the new connection. Same goes for pop3-accounts (your email settings), macro's you painfully added to normal.dot and all those nice little details.
Regarding the drivers: don't forget to install the motherboard drivers, if you have them, or maybe all the USB won't work anymore.
Don't underestimate this, if you don't have some experience.
Good luck,
Kees